For many years, the catalysts used in the polymerization of alpha-olefins have been the so-called Ziegler-Natta catalysts - for example, conventional, unsupported catalysts consisting mainly of the reaction products of a titanium compound, such as titanium trichloride, and a trialkylaluminum or an alkylaluminum halide. More recently, however, new Ziegler-Natta catalysts have been developed that are much more active than these conventional, unsupported catalysts. These new catalysts, called "high activity catalysts," comprise, for example, a titanium halide supported on other crystals such as magnesium chloride, and an alkylaluminum compound. The latter can be present as a complex with an electron donor compound. These high-activity catalysts have been described in the patent literature (e.g., in U.S. Pat. Nos. 3,830,787, 3,953,414, 4,051,313, 4,115,319, and 4,149,990).
The new high activity catalysts are especially useful in the homopolymerization of propylene, as well as in the copolymerization of a mixture of propylene and another alpha-olefin, such as ethylene. This is particularly so when the polymerization is carried out in a liquid diluent - for example, liquid propylene monomer as used in "liquid pool" polymerization. The higher efficiency catalysts have provided great increases in the amount of polymer produced per unit weight of catalyst, and in the yield of the desired isotactic polymer as compared to the unwanted atactic stereoisomer. As a result of the improvements provided by the high activity catalysts, the polymers produced contain such small amounts of residual catalyst that the conventional deashing steps can be eliminated.
Therefore, it is common practice to leave the catalyst residues in the final polymer product, which is commonly blended with other additives and pelletized for shipment to the users of the polymer. However, the residual catalyst must be neutralized and deactivated. Not only is it corrosive because of its acidic nature, but it is also still highly active and can react with other functional additives that may subsequently be blended with the polymer. In some cases, the reaction with the additives will produce highly colored compounds, thus discoloring the polymer pellets. The reactivity of the catalyst residues can also interfere with the intended effects of the additives used at this stage or in later processing.
Several different materials can be blended with the polymer to neutralize the acidity. These include certain alkaline earth salts and other alkaline inorganic compounds or minerals. However, while such additions can eliminate the corrosivity of the residues, the catalyst still remains active and can have detrimental reactions with other additives. Therefore, it is necessary to use an additional material, along with the neutralizer, to deactivate the catalyst.
Many catalyst deactivation methods have been disclosed, including the use of water, oxygen, peroxide solutions, alcohols, carbon monoxide, alkylene oxides, and the like. Examples of such methods are found in U.S. Pat. Nos. 3,435,019, 3,496,156, 4,029,877, 4,156,075, 4,167,619, 4,234,716, 4,314,053, 4,420,609, and 4,591,633. However, these all have certain disadvantages.
For example, alkylene oxides have been widely used for this purpose, but they cause undesirable off-odors in products molded from the polyolefin. The gaseous materials are generally too slow in their action and require special equipment for treating the polymer. Some of the peroxides proposed require the use of aqueous solutions, and it is not desirable to introduce significant amounts of water into these polymer systems. Some of the earlier methods in the prior art also were intended for use with the older catalyst systems and required the subsequent deashing of the polymer.
It is therefore an object of the present invention to provide a new, more efficient method of treatment of the polyolefins produced with high activity catalysts that obviates the disadvantages of the prior art. It is another object of the invention to provide a method in which both the neutralization of the catalyst residues and the chemical conversion of these residues to non-reactive substances will be accomplished by the addition of a single chemical substance, and such treatment will not require the deashing of the polymer.
Other objects and advantages of this invention will become apparent from a reading of the specification and appended claims.